The trial is based on preliminary data showing that upright tilt table testing can provoke a drop in blood pressure consistent with neurally mediated hypotension (NMH) in a high proportion of those with chronic fatigue syndrome (CFS), and that unblinded treatment of the NMH leads to an improvement in CFS symptoms in 40-70% of CFS patients. The specific aim of this study is to determine whether patients aged 18-50 years with CFS and NMH will have a greater improvement in (1) self- reported general sense of well being and (2) objective orthostatic tolerance when treated with fludrocortisone than when treated with placebo. Eligible subjects are randomized to receive either fludrocortisone (escalating to 0.1 mg/day) or placebo for nine weeks. In week 8-9 of treatment, subjects undergo repeat tilt testing. The primary outcome measure is the proportion with a 15 point improvement in wellness on a 100 point wellness score, and a secondary outcome is the proportion with improvement in the number of minutes before the development of hypotension during upright tilt. The first patient was studied as part of this protocol March 4, 1996. From that date until November 30, 1996, we evaluated 70 patients with chronic fatigue syndrome. In 36 of these (54%), hypotension was documented during the first two stages of upright tilt testing. One patient with hypotension during the test had a wellness score that was above 70 at the time of the positive tilt test, and was therefore ineligible for participation. The remaining 35 subjects were randomized to fludrocortisone or placebo. Of these, 4 subjects have dropped out due to side effects. The study procedures appear to be well tolerated, and there have been no unanticipated problems with tilt testing or with the study medications. With very few exceptions, the study outcome data have been received in a timely manner by the company coordinating data entry. Of those enrolled, 25(71%) are female. All are Caucasian. These demographic characteristics reflect the epidemiologic patterns seen in other studies of CFS. Only one back individual has been evaluated for participation, but her tilt test was incomplete; no Hispanic, Asian or Pacific Islander, or American Indian or Alaskan Native has applied to the study.